The present invention relates to a steerable bottom hole assembly including a rotary bit powered by a positive displacement motor or a rotary steerable device. The bottom hole assembly of the present invention may be utilized to efficiently drill a deviated borehole at a high rate of penetration.
Steerable drilling systems are increasingly used to controllably drill a deviated borehole from a straight section of a wellbore. In a simplified application, the wellbore is a straight vertical hole, and the drilling operator desires to drill a deviated borehole off the straight wellbore in order to thereafter drill substantially horizontally in an oil bearing formation. Steerable drilling systems conventionally utilize a downhole motor (mud motor) powered by drilling fluid (mud) pumped from the surface to rotate a bit. The motor and bit are supported from a drill string that extends to the well surface. The motor rotates the bit with a drive linkage extending through a bent sub or bent housing positioned between the power section of the motor and the drill bit. Those skilled in the art recognize that the bent sub may actually comprise more than one bend to obtain a net effect which is hereafter referred to for simplicity as a xe2x80x9cbendxe2x80x9d and associated xe2x80x9cbend angle.xe2x80x9d The terms xe2x80x9cbendxe2x80x9d and xe2x80x9cbend anglexe2x80x9d are more precisely defined below.
To steer the bit, the drilling operator conventionally holds the drill string from rotation and powers the motor to rotate the bit while the motor housing is advanced (slides) along the borehole during penetration. During this sliding operation, the bend directs the bit away from the axis of the borehole to provide a slightly curved borehole section, with the curve achieving the desired deviation or build angle. When a straight or tangent section of the deviated borehole is desired, the drill string and thus the motor housing are rotated, which generally causes a slightly larger bore to be drilled along a straight path tangent to the curved section. U.S. Pat. No. 4,667,751, now RE 33,751, is exemplary of the prior art relating to deviated borehole drilling. Most operators recognize that the rate of penetration (ROP) of the bit drilling through the formation is significantly less when the motor housing is not rotated, and accordingly sliding of the motor with no motor rotation is conventionally limited to operations required to obtain the desired deviation or build, thereby obtaining an overall acceptable build rate when drilling the deviated borehole. Accordingly, the deviated borehole typically consists of two or more relatively short length curved borehole sections, and one or more relatively long tangent sections each extending between two curved sections.
Downhole mud motors are conventionally stabilized at two or more locations along the motor housing, as disclosed in U.S. Pat. No. 5,513,714, and WO 95/25872. The bottom hole assembly (BHA) used in steerable systems commonly employs two or three stabilizers on the motor to give directional control and to improve hole quality. Also, selective positioning of stabilizers on the motor produces known contact points with the wellbore to assist in building the curve at a predetermined build rate.
While stabilizers are thus accepted components of steerable BHAs, the use of such stabilizers causes problems when in the steering mode, i.e., when only the bit is rotated and the motor slides in the hole while the drill string and motor housing are not rotated to drill a curved borehole section. Motor stabilizers provide discrete contact points with the wellbore, thereby making sliding of the BHA difficult while simultaneously maintaining the desired WOB. Accordingly, drilling operators have attempted to avoid the problems caused by the stabilizers by running the BHA xe2x80x9cslick,xe2x80x9d i.e., with no stabilizers on the motor housing. Directional control may be sacrificed, however, because the unstabilized motor can more easily shift radially when drilling, thereby altering the drilling trajectory.
Bits used in steerable assemblies commonly employ fixed PDC cutters on the bit face. The total gauge length of a drill bit is the axial length from the point where the forward cutting structure reaches full diameter to the top of the gauge section. The gauge section is typically formed from a high wear resistant material. Drilling operations conventionally use a bit with a short gauge length. A short bit gauge length is desired since, when in the steering mode, the side cutting ability of the bit required to initiate a deviation is adversely affected by the bit gauge length. A long gauge on a bit is commonly used in straight hole drilling to avoid or minimize any build, and accordingly is considered contrary to the objective of a steerable system. A long gauge bit is considered by some to be functionally similar to a conventional bit and a xe2x80x9cpiggybackxe2x80x9d or xe2x80x9ctandemxe2x80x9d stabilizer immediately above the bit. This piggyback arrangement has been attempted in a steerable BHA, and has been widely discarded since the BHA has little or no ability to deviate the borehole trajectory. The accepted view has thus been that the use of a long gauge bit, or a piggyback stabilizer immediately above a conventional short gauge bit, in a steerable BHA results in the loss of the drilling operator""s ability to quickly change direction, i.e., they do not allow the BHA to steer or steering is very limited and unpredictable. The use of PDC bits with a double or xe2x80x9ctandemxe2x80x9d gauge section for steerable motor applications is nevertheless disclosed in SPE 39308 entitled xe2x80x9cDevelopment and Successful Application of Unique Steerable PDC Bits.xe2x80x9d
Most steerable BHAs are driven by a positive displacement motor (PDM), and most commonly by a Moineau motor which utilizes a spiraling rotor which is driven by fluid pressure passing between the rotor and stator. PDMs are capable of producing high torque, low speed drilling that is generally desirable for steerable applications. Some operators have utilized steerable BHAs driven by a turbine-type motor, which is also referred to as a turbodrill. A turbodrill operates under a concept of fluid slippage past the turbine vanes, and thus operates at a much lower torque and a much higher rotary speed than a PDM. Most formations drilled by PDMs cannot be economically drilled by turbodrills, and the use of turbodrills to drill curved boreholes is very limited. Nevertheless, turbodrills have been used in some steerable applications, as evidenced by the article xe2x80x9cSteerable Turbodrilling Setting New ROP Records,xe2x80x9d OFFSHORE, August 1997, pp. 40 and 42. The action of the PDC bit powered by a PDM is also substantially different than the action of a PDC bit powered by a turbodrill because the turbodrill rotates the bit at a much higher speed and a much lower torque.
Turbodrills require a significant pressure drop across the motor to rotate the bit, which inherently limits the applications in which turbodrills can practically be used. To increase the torque in the turbodrill, the power section of the motor has to be made longer. Power sections of conventional turbodrills are often 30 feet or more in length, and increasing the length of the turbodrill power section is both costly and adversely affects the ability of the turbodrill to be used in steerable applications.
A rotary steerable device (RSD) can be used in place of a PDM. An RSD is a device that tilts or applies an off-axis force to the bit in the desired direction in order to steer a directional well, even while the entire drillstring is rotating. A rotary steerable system enables the operator to drill far-more-complex directional and extended-reach wells than ever before, including particularly targets that previously were thought to be impossible to reach with conventional steering assemblies. A rotary steerable system may provide the operator and the engineers, geologists, directional drillers and LWD operators with valuable real-time, continuous steering information at the surface, i.e., where it is most needed. A rotary steerable automated drilling system is a technology solution that may translate into significant savings in time and money.
Rotary steerable technology is disclosed in U.S. Pat. No. 5,685,379, 5,706,905, 5,803,185, and 5,875,859, and also in Great Britain reference 2,172,324, 2,172,325, and 2,307,533. Applicant also incorporates by reference herein U.S. application Ser. No. 09/253,599 filed Jul. 14, 1999 entitled xe2x80x9cSteerable Rotary Drilling Device and Directional Drilling Method.xe2x80x9d
Automated, or self-correcting steering technology enables one to maintain the desired toolface and bend angle, while maximizing drillstring RPM and increasing ROP. Unlike conventional steering assemblies, the rotary steerable system allows for continuous rotation of the entire drillstring while steering. Steering while sliding with a PDM is typically accompanied by significant drag, which may limit the ability to transfer weight to the bit. Instead, a rotary steerable system is steered by tilting or applying an off-axis force at the bit in the direction that one wishes to go while rotating the drillpipe. When steering is not desired, one simply instructs the tool to turn off the bit tilt or off-axis force and point straight. Since there is no sliding involved with the rotary steerable system, the traditional problems related to sliding, such as discontinuous weight transfer, differential sticking and drag problems, are greatly reduced. With this technology, the well bore has a smooth profile as the operator changes course. Local doglegs are minimized and the effects of tortuosity and other hole problems are significantly reduced. With this system, one optimizes the ability to complete the well while improving the ROP and prolonging bit life.
A rotary steerable system has even further advantages. For instance, hole-cleaning characteristics are greatly improved because the continuous rotation facilitates better cuttings removal. Unlike positive differential mud motors, this system has no traditional, elastomer motor power section, a component subject to wear and environmental dependencies. By removing the need for a power section with the rotary steerable system, torque is coupled directly through the drillpipe from the surface to the bit, thereby resulting in potentially longer bit runs. Plus, this technology is compatible with virtually all types of continuous fluid mud systems.
Those skilled in the art have long sought improvements in the performance of a steerable BHA which will result in a higher ROP, particularly if a higher ROP can be obtained with better hole quality and without adversely affecting the ability of the BHA to reliably steer the bit. Such improvements in the BHA and in the method of operating the BHA would result in considerable savings in the time and money utilized to drill a well, particularly if the BHA can be used to penetrate farther into the formation before the BHA is retrieved to the surface for altering the BHA or for replacing the bit. By improving the quality of both the curved borehole sections and the straight borehole sections of a deviated borehole, the time and money required for inserting a casing in the well and then cementing the casing in place are reduced. The long standing goal of an improved steerable BHA and method of drilling a deviated borehole has thus been to save both time and money in the production of hydrocarbons.
An improved bottom hole assembly (BHA) is provided for controllably drilling a deviated borehole. The bottom hole assembly may include either a positive displacement motor (PDM) driven by pumping downhole fluid through the motor for rotating the bit, or the BHA may include a rotary steerable device (RSD) such that the bit is rotated by rotating the drill string at the surface. The BHA lower housing surrounding the rotating shaft is preferably xe2x80x9cslickxe2x80x9d in that it has a substantially uniform diameter housing outer surface without stabilizers extending radially therefrom. The housing on a PDM has a bend. The bend on a PDM occurs at the intersection of the power section central axis and the lower bearing section central axis. The bend angle on a PDM is the angle between these two axes. The housing on an RSD does not have a bend. The bend on an RSD occurs at the intersection of the housing central axis and the lower shaft central axis. The bend angle on an RSD is the angle between these two axes. The bottom hole assembly includes a long gauge bit, with the bit having a bit face having cutters thereon and defining a bit diameter, and a long cylindrical gauge section above the bit face. The total gauge length of the bit is at least 75% of the bit diameter. The total gauge length of a drill bit is the axial length from the point where the forward cutting structure reaches full diameter to the top of the gauge section. At least 50% of the total gauge length is substantially full gauge. Most importantly, the axial spacing between the bend and the bit face is controlled to less than twelve times the bit diameter.
According to the method of the invention, a bottom hole assembly is preferably provided with a slick housing having a uniform diameter outer surface without stabilizers extending radially therefrom. The bit is rotated at a speed of less than 350 rpm. The bit has a gauge section above the bit face such that the total gauge length is at least 75% of the bit diameter. At least 50% of the total gauge length is substantially full gauge. The axial spacing between the bend and the bit face is controlled to less than twelve times the bit diameter. When drilling the deviated borehole, a low WOB may be applied to the bit face compared to prior art drilling techniques.
It is an object of the present invention to provide an improved BHA for drilling a deviated borehole at a high rate of penetration (ROP) compared to prior art BHAs. This high ROP is achieved when either the PDM or the RSD is used in the rotation of the bit.
It is a related object of the invention to form a deviated borehole with a BHA utilizing improved drilling methods so that the borehole quality is enhanced compared to the borehole quality obtained by prior art methods. The improved borehole quality, including the reduction or elimination of borehole spiraling, results in higher quality formation evaluation logs and subsequently allows the casing or liner to be more easily slid through the deviated borehole.
It is an object of the present invention to provide an improved bottom hole assembly for drilling a deviated borehole, with the bottom hole assembly including a rotary shaft having a lower central axis offset at a selected bend angle from an upper central axis by a bend, a housing having a substantially uniform diameter outer surface enclosing a portion of the rotary shaft, and a long gauge bit powered by the rotary shaft. The long gauge bit has a bit face defining a bit diameter and a gauge section having a substantially uniform diameter cylindrical surface spaced above the bit face, with a total gauge length of at least 75% of the bit diameter. At least 50% of the total gauge length is substantially full gauge.
Another object of the invention is to provide an improved method of drilling a deviated borehole utilizing a bottom hole assembly which includes a rotary shaft having a lower central axis offset at a selected bend angle from an upper central axis by a bend, wherein the bottom hole assembly further includes a bit rotated by the rotary shaft and the method includes providing a housing having a substantially uniform diameter outer surface surrounding the rotary shaft upper axis, providing a long gauge bit having a gauge section with a substantially uniform diameter cylindrical surface and with a total gauge length of at least 75% of the bit diameter, at least 50% of the total gauge length being substantially full gauge, and rotating the bit at a speed of less the 350 rpm to form a curved section of the deviated borehole. A method of the present invention may be used with either a positive displacement motor (PDM) or with a rotary steerable device (RSD).
Another object of the present invention is to provide an improved bottomhole assembly for drilling a deviated borehole with a long gauge bit having a gauge section wherein the portion of the total gauge length that is substantially full gauge has a centerline, that centerline preferably having a maximum eccentricity of 0.03 inches relative to the centerline of the rotary shaft. This method may also be obtained by taking special precautions with respect to the use of a conventional bit and a piggyback stabilizer. An improved method of drilling a deviated borehole according to the present invention includes providing a bottomhole assembly that satisfies the above relationship.
Yet another object of this invention is to provide a bottom hole assembly for drilling a deviated borehole, wherein the long gauge bit is powered by rotating the shaft, and one or more sensors positioned substantially along the total gauge length of the long gauge bit or elsewhere in the BHA for sensing selected parameters while drilling. Signals from these sensors may then be used by the drilling operator to improve the efficiency of the drilling operation. According to the related method, information from the sensors may be provided in real time to the drilling operator, and the operator may then better control drilling parameters such as weight on bit while rotating the bit at a speed of less than 350 rpm to form a curved section of the deviated borehole.
Still another object of the invention is to provide an improved bottom hole assembly for drilling a deviated borehole, wherein the rotary shaft which passes through the bend is rotated at the surface. A long gauge bit is provided with a gauge section such that the total gauge length is at least 75% of the bit diameter and at least 50% of the total gauge length is substantially full gauge. The axial spacing between the bend and the bit face is less than twelve times the bit diameter. According to the related method of this invention, the drilling operator is able to improve drilling efficiency while rotating the bit at a speed of less than 350 rpm to form a curved section of the deviated borehole.
It is a feature of the invention to provide a method for drilling a deviated borehole wherein the weight-on-bit (WOB) as measured at the surface is substantially reduced and more consistent compared to prior art systems by eliminating the drag normally attributable to conventional BHAs.
Another feature of the invention is a method of drilling a deviated borehole wherein a larger portion of the deviated borehole may be drilled with the motor sliding and not rotating compared to prior art methods. The length of the curved borehole sections compared to the straight borehole sections may thus be significantly increased. The bit may also be rotated from the surface, with a bend being provided in an RSD.
Another feature of the invention is that hole cleaning is improved over conventional drilling methods due to improved borehole quality.
It is also a feature of the invention to improve borehole quality by providing a BHA for powering a long gauge bit which reduces bit whirling and hole spiraling. A related feature of the invention achieves a reduction in the bend angle to reduce both spiraling and whirling. The reduced bend angle in the housing of a PDM reduces stress on the housing and minimizes bit whirling when drilling a straight tangent section of the deviated borehole. The reduced bend BHA nevertheless achieves the desired build rate because of the short distance between the bend and the bit face.
It is a feature of the present invention that a bottom hole assembly may have an axial spacing between the bend and the bit face of less than twelve times the bit diameter. A related feature of this invention is that this reduced spacing may be obtained in part by providing a pin connection at a lowermost end of the rotary shaft and a mating box connection at the uppermost end of a long gauge bit.
Another feature of the invention is that the axial spacing between the bend and the bit face may be held to less than twelve times the bit diameter, and the bend may be less than 0.6 degrees when using a RSD.
Still another feature of this invention is that the axial spacing between the bend and the bit face may be held to less than twelve times the bit diameter, with the bend being less than 1.5 degrees in a PDM. The motor housing may be rotated with the drill pipe to form a straight section of a deviated borehole.
Still another feature of this invention is that the bottom hole assembly may be provided with one or more downhole sensors positioned substantially along the length of the total gauge length or elsewhere in the BHA for sensing any desired borehole parameter.
Yet another feature of the present invention is that improved techniques may be used with a PDM, so that the method includes rotating the motor housing within the borehole to rotate the bit when forming a straight section of the deviated borehole.
The improved method of the invention preferably includes controlling the actual weight on the bit such that the bits face exerts less than about 200 pounds axial force per square inch of the PDC bit face cross-sectional area.
According to the method of this invention, the bend may be maintained to less than 1.5 degrees when using a PDM, and a bit may be rotated at less than 350 rpm.
Yet another feature of the invention is that the one or more sensors may be provided substantially along the total gauge length of the bit and/or bit and stabilizer. These sensors may include a vibration sensor and/or a rotational sensor for sensing the speed of the rotary shaft.
Still another feature of this invention is that an MWD sub may be located above the motor, and a short hop telemetry system may be used for communicating data from the one or more sensors in real time to the MWD sub. The short hop telemetry system may be either an acoustic system or an electromagnetic system.
Yet another feature of the invention is that data from the sensors may be stored within the total gauge length of the long gauge bit and then output to a computer at the surface.
Still another feature of the invention is that the output from the one or more sensors provides input to the drilling operator either in real time or between bit runs, so that the drilling operator may significantly improve the efficiency of the drilling operation and/or the quality of the drilled borehole.
It is an advantage of the present invention that the spacing between the bend in a PDM or RSD and the bit face may be reduced by providing a rotating shaft having a pin connection at its lowermost end for mating engagement with a box connection of a long gauge bit. This connection may be made within the long gauge of the bit to increase rigidity.
Another advantage of the invention is that a relatively low torque PDM may be efficiently used in the BHA when drilling a deviated borehole. Relatively low torque requirements for the motor allow the motor to be reliably used in high temperature applications. The low torque output requirement of the PDM may also allow the power section of the motor to be shortened.
A significant advantage of this invention is that a deviated borehole is drilled while subjecting the bit to a relatively consistent and low actual WOB compared to prior art drilling systems. Lower actual WOB contributes to a short spacing between the bend and the bit face, a low torque PDM and better borehole quality.
It is also an advantage of the present invention that the bottom hole assembly is relatively compact. Sensors provided substantially along the total gauge length may transmit signals to a measurement-while-drilling (MWD) system, which then transmits borehole information to the surface while drilling the deviated borehole, thus further improving the drilling efficiency.
A significant advantage of this invention is that the BHA results in surprisingly low axial, radial and torsional vibrations to the benefit of all BHA components, thereby increasing the reliability and longevity of the BHA.
Still another advantage of the invention is that the BHA may be used to drill a deviated borehole while suspended in the well from coiled tubing.
Yet another advantage of the present invention is that a drill collar assembly may be provided above the motor, with a drill collar assembly having an axial length of less than 200 feet.
Another advantage of this invention is that when the techniques are used with a PDM, the bend may be less than about 1.5 degrees. A related advantage of the invention is that when the techniques are used with a RSD, the bend may be less than 0.6 degrees.
These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings.